Process for producing electrolytic manganese dioxide

ABSTRACT

This invention relates generally to processes for extracting iron and/or calcium from geothermal brines.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/763,357, filed Jan. 23, 2004, which is a continuation of U.S. patentapplication Ser. No. 10/160,809, filed May 31, 2002, which issued asU.S. Pat. No. 6,682,644. Each of these applications is incorporated byreference herein.

TECHNICAL FIELD

The present invention generally relates to a process for extractingmanganese from geothermal brines. More particularly, the presentinvention relates to a liquid-liquid extraction method for separatingmanganese dioxide form brine using an electrolytic process.

BACKGROUND ART

Manganese in many forms is used in a variety of industrial and otherapplications. It is the fourth most used metal in terms of tonnage,after iron, aluminum and copper. As a trace element, it is found in thebody, and also has a variety of uses in industry. For example, manganesedioxide is used in dry cell batteries, in aluminum cans, and in theelectronic components of television sets.

Manganese is most often extracted from seawater and other naturalsources, but can also be separated from other metals found in aqueoussolutions that are byproducts of many industrial processes. For example,geothermal steam and hot brines are found in naturally occurring, largesubterranean reservoirs throughout the world. In many areas whereextraction is convenient, the steam and hot brines provide a partiallyrenewable resource for the production of power. The pressurized, hotgeothermal brines are extracted from the earth to generate power byusing steam flashed off from the brine to power a turbine. Thereafter,metals such as manganese can be extracted from the brine before it isreturned to the ground.

One of the general problems encountered with the extraction of metalsfrom aqueous solutions involves changes in pH associated with theexchange of metal ions for hydrogen ions in ion exchange reactions. Thiscauses a progressive lowering of the pH which in turn impedes theefficiency of the process. Attempts to solve this problem have beenreported, but their success has been limited. See, for example, U.S.Pat. No. 4,128,493, which reports the use of organic solvents andquaternary ammonium salts to extract metals from acidic solutions.

Other methods for recovering metals such as zinc from geothermal brineinclude precipitation with sulfides and various combinations of solventextractions and electromagnetic stripping. However, continuous batchprocesses using these methods are limited due to scaling of theequipment due to the presence of large amounts of silica.

Manganese is usually extracted in the form of electrolytic manganesedioxide (EMD). Perhaps the most common process being used today forextracting manganese is by mixing the manganese-containing material withsulfuric acid to form a manganese sulfate electrolyte. This intermediateis separated from other metals by precipitation and filtration.Thereafter, the manganese sulfate is subjected to solvent extraction andelectrowinning. See, for example, PCT WO 99/14403; and L. A. Mel'nik, etal., Russian J. of Electrochemistry 32: 248-51 (1996).

The current sulfate process for the production of electrolytic manganesedioxide (EMD) was invented more than seventy years ago in the US.However, it was commercialized in the early 1950's in a move largelydriven by the US military seeking higher quality batteries for use inKorea. The standard sulfate process is still the only commercial processfor the manufacture of EMD.

The only significant market for EMD is its use in dry cell batteries(small amounts are also consumed in the production of soft ferrites forthe electronics industries). Optimum battery performance is based on acombination of chemical, physical and electrical characteristics.However, the key feature of EMD which sets it apart from other manganesedioxides is its crystal structure. This all important characteristic isdeveloped during high temperature aqueous electrolysis, the heart of theEMD manufacturing process. Although a typical specification for EMDmight have about 20 components, EMD quality is defined by 4 keycriteria: Crystal structure (disordered, hydrated, non-stoichiometric);Chemical purity (minimum 92% MnO₂ (remainder essentially water) with keyimpurities at the single digit ppm level); Density (higher the better,since batteries are fixed volume devices); and Intrinsic dischargecapacity (measured in mAh/g MnO2, again higher the better).

The standard sulfate process for EMD has been undergoing continualdevelopment since it was first commercialized in the 1950s. See e.g.,Nathsarma, et al., Hydrometallurgy 45: 169-79 (1997); Alexperov et al.,Journal of Applied Chemistry of the USSR 65: 2342-44 (1992). Thesedevelopments have been largely driven by demands for improvements inproduct quality. However, the key elements of the process are unchanged.

One of the largest challenges facing the EMD industry is wastemanagement. The best manganese ores available to the industry containonly 50% Mn. Insoluble gangue from the ore combined with wet filtercakes from process purification and filtration stages typically generatesome 2 to 3 tons of solid waste per ton of EMD product. While manganeseoccurs widely in nature and is not generally considered a toxic element,solid wastes containing soluble manganese must be immobilized andcontained in sealed dump sites to prevent ground water contamination. Alarge proportion of the world's EMD capacity is located inenvironmentally sensitive regions, such as Japan, Europe, USA andAustralia. Containment of waste is a major limitation to expansion formany existing producers.

A second limitation of the standard sulfate process is the low currentdensity, typically about 55 A/m², at which plants must operate, althoughsome plants operate between about 50 A/m² to 70 A/m². This is one fifthto one tenth the current density normally associated with metalelectrowinning processes. It is known that a chloride electrolytesupports higher current densities for the production of EMD, in theorder of 80 to 100 A/m², improving plant productivity and reducingcapital cost per annual product ton. However, a chloride electrolytesystem has not yet been adopted.

Accordingly, there is a need for a more efficient process for extractingEMD from various natural and industrial sources that is more compatiblewith environmental concerns and commercial needs. The present process,based on the recovery of manganese units from liquid brine andelectrolysis of a chloride liquor, has the potential to overcome orminimize these two limitations of the standard sulfate process.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 presents a flow chart depicting a particular embodiment of thedisclosed process involving iron removal followed by calcium removalfrom a brine solution.

FIG. 2 presents a flow chart depicting another particular embodiment ofthe disclosed process involving impurity removal of iron and calciumafter primary solvent extraction.

FIG. 3 presents a flow chart depicting yet another particular embodimentof the disclosed process involving multiple scrubbing steps utilizingspent electrolyte and pH adjustment.

FIG. 4 presents a flow chart depicting yet another particular embodimentof the disclosed process involving multiple scrubbing steps utilizingspent electrolyte pH and EMF adjustment.

FIG. 5 presents a flow chart depicting yet another particular embodimentof the disclosed process involving iron removal prior to QL loading.

FIG. 6 presents a schematic flowsheet depicting a particular aspect ofiron oxidation and precipitation in cascade-arranged tanks.

FIG. 7 presents a schematic of the process steps that may be used foriron removal.

DISCLOSURE OF THE INVENTION

The presently described process involves manganese extraction fromaqueous brine solutions which may contain iron. The presently describedprocess may also involve extracting manganese from aqueous brinesolutions which may contain calcium and/or zinc in addition to iron. Thepresently described processes further involve methods of producingelectrolytic manganese from manganese chloride.

In a particular aspect, the invention utilizes the implementation of thechloride route to electrolytic manganese dioxide (EMD). There areindications that EMD made from chloride solution might beelectrochemically superior to EMD made from the standard sulfate route.In addition, the chloride process operates at higher current densitieswhen compared to the sulfate system and this will represent savings inboth operating and capital costs. Most geothermal brines are chloridesolutions and this makes the chloride route straightforward. However, amethod for conversion to the sulfate route is provided if processconditions necessitate a switch from chloride to sulfate.

According to a first aspect of the present invention, a method isprovided wherein manganese is extracted from an aqueous solutioncontaining iron, which solution may be a geothermal brine, comprisingthe steps of: extracting the manganese and iron by contacting theaqueous solution with a QL reagent, wherein the QL reagent comprises aquaternary ammonium compound, a hydrogen ion exchange reagent and anorganic solvent, such that an organic phase containing the manganese andiron and an aqueous phase are formed; stripping the manganese and ironfrom the organic phase by contacting the organic phase with acid, suchthat the manganese and iron shift from the organic phase to the aqueousphase; removing the iron from the aqueous phase by oxidizing the iron,such that the iron precipitates leaving a supernatant containing themanganese; and electrowinning the supernatant, such that electrolyticmanganese dioxide deposits on the anode.

In one embodiment of this aspect of the present invention, the pH of thesolution remains constant. An exemplary constant pH of this solution isin the range of about 1 to about 5, about 2 to about 4, and preferablyabout 1.5 to about 3.

In another embodiment, there is provided a method for manganeseextractions where calcium is extracted by introducing manganese-richstrip solution to the organic phase of the manganese and iron extractionstep, displacing calcium from the organic phase, and scrubbing thedisplaced calcium from the solution.

The manganese-rich strip solution described above may be comprised of aside-stream of recycled electrolyte. Further, this solution may containan organic phase/aqueous phase (O/A) ratio between about 5-20. In oneembodiment, the O/A ratio is about 10.

The method described above may also include neutralizing the organicphase during stripping and washing to reform the components of the QLreagent. This generally involves contacting the organic phase with asolution comprised of sodium hydroxide, allowing the aqueous phase andorganic phase to separate, discarding the aqueous phase, and then addingdi-butyl butyl phosphonate (DBBP) to the organic phase.

In a particular embodiment, during the reformation of the QL reagentdescribed above, the organic phase and sodium hydroxide solution arecontacted in an O/A ratio of 1. In a further embodiment of the QLreformation, the organic phase is contacted with a sodium chloridesolution or Na₂SO₄ solution.

In a particular embodiment of the present invention, a method isprovided wherein the supernatant of the oxidation step may comprisemanganese chloride.

In one embodiment, the extraction of manganese and iron may be performedin at least one column comprising a closed and pressurized vesicle withfillers contained therein. There may be multiple columns which may beconnected in a series such that the product of one column may betransferred to the next for further extraction. These columns maypreferably be maintained at a positive air pressure with nitrogen,another inert gas or steam.

In a still further embodiment, all components of the extraction step(described above) are performed under anoxic conditions.

The organic solvent is preferably selected from the group comprising analiphatic or aromatic hydrocarbon. This organic solvent may preferablybe a chlorinated hydrocarbon. In a still further embodiment, the organicsolvent may be heptane.

In another embodiment, the quaternary ammonium compound is tricaprylmethylammonium chloride (aliquot 336). The hydrogen exchange reagent ispreferably diethylhexylphosphate (DEPHA).

The acid used in the extraction step is preferably a non-oxidizing acid.A still further embodiment includes hydrochloric acid or sulfuric acidas non-oxidizing acids.

In addition, during the oxidation step, the pH of the aqueous phase isusually between 2-5.

In another aspect of the method provided herein, iron is oxidized withan oxidizing agent, such as sodium hypochlorite, sodium peroxide,hydrogen peroxide, and chlorine gas. The oxidizing agent may also becombined with a buffer such as sodium carbonate.

In another aspect of the invention, the supernatant from theelectrowinning step may be combined with an acid in equal parts toproduce an electrolyte bath. The acid may preferably be sulfuric acid orhydrochloric acid.

In yet another aspect of the invention, there is provided a method forextracting manganese from an aqueous solution containing iron,comprising the steps of: obtaining a zinc and calcium depletedhydrochloric acid solution containing manganese and iron; removing theiron from the solution by oxidizing the iron, such that the ironprecipitates leaving a supernatant containing the manganese in the formof manganese chloride; and electrowinning the supernatant in ahydrochloric acid bath, such that electrolytic manganese dioxidedeposits on the anode.

DETAILED DESCRIPTION

The present invention relates to a process for recovering electrolyticmanganese dioxide (EMD) from geothermal brine or from an aqueoussolution containing manganese and iron and other components such as, butnot limited to, calcium and/or zinc. This process involves solventextraction to recover manganese from liquid brine, followed byelectrolysis of a chloride (or sulfate) liquor. For most commercialuses, EMD should have four basic characteristics; the desired crystalstructure, sufficient chemical purity, sufficient density, and adequateintrinsic discharge capabilities.

Extraction of zinc from brine is usually performed prior to theextraction of manganese through classic ion exchange or solventextraction. Accordingly, two of the principle obstacles for recoveringmanganese from zinc-depleted brine are the separation of manganese fromiron and calcium, and finding an extractant which does not lower the pHof the brine as manganese is extracted. A constant pH is preferredbecause, as the pH of brine decreases, extraction efficiency formanganese also decreases until, at a pH of about 1.5, manganeseextraction is effectively zero.

Process Overview

The process of the present invention can be described in terms ofindividual process steps as follows:

-   -   Extraction: Liquid-liquid anoxic extraction of manganese (and        iron) from brine using QL reagent and, optionally, spent        electrolyte solution. This may include the sub-processes of        loading, scrubbing and stripping.    -   Oxidation: oxidation to cause the iron to precipitate, which is        thereafter removed.    -   Electrowinning: electrolysis to extract the EMD which is        deposited on the cathode.

As the above elements are described by their principal purposes, it isimportant to note that the order presented is not necessarily the onlyor even the preferred order in which they may be performed. Further,each step may involve any one, or combination, of a number ofsubprocesses and the steps and subprocesses may overlap and run intoeach other. Given any particular embodiment, the process steps maychange. Therefore, unless otherwise noted, the presentation of the stepsbelow is provided in a particular order only for ease of understandingand clarity of presentation.

Extraction

As provided below, the extraction step may involve three sub processes:loading, scrubbing and stripping. Each subprocess may be multiplyperformed in a particular embodiment of the present invention or notperformed at all. However, the goal of the steps is to remove impuritiesor contaminants from the brine solution to aid in obtaining purifiedmanganese therefrom. In a particular embodiment, the geothermal brinethat may be used as a starting material is depleted of zinc prior to usein the present processes. Zinc depletion preferably leaves iron andcalcium as the primary contaminants in the brine in addition tomanganese.

Loading

In one aspect of the extraction step, zinc-depleted and/or iron-depletedbrine is contacted with an organic phase QL reagent in a suitablevesicle that can be maintained under anoxic conditions. Anoxicconditions are preferred to prevent the premature precipitation of ironfrom solution, and the resulting contamination of the solution, due tooxidation of Fe²⁺ to Fe³⁺. These conditions have the additionaladvantage of preventing the formation of ferric silicate in solution,which would potentially inhibit or complicate the extraction process.

Preferably, the vesicle described above is a column that allows thebrine and QL reagent to be mixed together and maintained in a closedenvironment throughout the extraction process. Such columns arecommercially available from, e.g., Koch Process Technologies, Inc.,Parsippany, N.J. (Koch columns). The columns may preferably contain“fillers” which are directed to enhancing mass transfer. Suitable columnfillers include inert particles and/or other solid matter that does notinterfere with the extraction process. The fillers are more preferablymoveable reciprocating plates.

Therefore, liquid-liquid extraction Koch columns with reciprocatingplates may be used as the principle process equipment in separatingmanganese from the brine. The Koch columns are multistage contactdevices which are more efficient when compared to mixer settler units(which are also contemplated), the dominant type of equipment used inhydrometallurgy. It would require multiple mixer settlers arranged in aseries to accomplish the optimal separation, described below, which caneffectively be performed in only one of the presently described columns.However, arrangement of a series of Koch columns with reciprocatingplates are also contemplated in the present invention to obtain optimumextraction. Suitable columns are sealed pressure vessels which prohibitcontact with air (i.e., maintaining anoxic conditions) to avoidpremature oxidation of iron. An anoxic environment may be maintainedthrough positive pressure with an inert gas such as nitrogen, or withsteam from the geothermal brine solution. Positive pressure refers tomaintenance of a pressure within the column above atmospheric pressure.Further, suitable equipment is required which operates effectively athigh temperatures, often between about 180° F. to about 250° F., about230° F., or preferably about 225° F., and avoids significant evaporativeloss due to the temperature and pressure. The pressurized liquid-liquidextraction columns described above may be used to accomplish this task.Similar columns are suitable for performing the loading, scrubbing andstripping steps described below.

A preferred extracting compound of the present invention is “QLreagent.” (See U.S. Pat. No. 4,128,493.) The QL reagent is a combinationof two commercially available chemicals: a quaternary ammonium cation,such as Aliquot 336 (i.e., tricapryl methylammonium chloride) (“Q”),Aldrich Chemical Co., Milwaukee, Wis., and a deprotonated anion, such asDEHPA (i.e., diethylhexylphosphate) (“L”), in a suitable organicsolvent. The QL reagent is prepared using known methods. For example,see methods described in U.S. Pat. No. 4,128,493. As described therein,an example of the basic reaction of the QL reagent with a metal salt(MX) can be represented as:

I. 2QL_(org).+MX_(aq).→L₂M_(org).+Q₂X_(org).

II. M²⁺+2QL_(org).+2X_(aq).→L₂M_(org).+2QX_(aq).

(I) where M is a divalent cation and X is a divalent anion (e.g.,sulfate), or (II) where X is a monovalent anion (e.g., chloride).According to either of these equations, there is no net transfer ofhydrogen ions from the organic phase to the aqueous phase duringextraction. Accordingly, there is no appreciable change in pH. Thereagent exists as an ion pair in the organic phase, and extracts Mn, Caand Fe as the divalent chloride salts (MnCl₂, CaCl₂ and FeCl₂) at aself-buffering pH of about 3.5. The reagent extracts iron and manganeseapproximately equally and is selective against calcium. However, insolutions with excess concentrations of calcium, the loaded organicphase may contain equal concentrations of all three metals.

Without being bound by theory, suitable quaternary ammonium cations(“Q”) for use in preparing the QL reagent may be represented by thefollowing formula:

where R₁ is a hydrocarbon radical such as alkyl, alkenyl, aryl, alkaryl,arylalkyl and the like of approximately 6 to 24 carbon atoms, where R₂,R₃ and R₄ are hydrocarbon radicals of 1 to 24 carbon atoms.Representative anions X⁻ and X²⁻ are chloride, bromide, iodide, sulfate,bisulfate.

Suitable quaternary ammonium cations include lauryltrimethyl ammoniumchloride, myristyltrimethyl ammonium chloride, palmityltrimethylammonium chloride, lauryltrimethyl ammonium sulfate, myristyltrimethylammonium bromide, palmityltrimethyl ammonium iodide, stearyltrimethylammonium chloride, stearyltrimethyl ammonium sulfate, oleyltrimethylammonium chloride, oleylbutyldimethyl ammonium sulfate, dilauryldimethylammonium chloride, distearyldimethyl ammonium sulfate, trilaurylmethylammonium chloride, tioctylmethyl ammonium bromide, tridecylmethylammonium chloride, stearylbenzyldimethyl ammonium sulfate,oleylbenzyldiethyl ammonium chloride and the like. In one embodiment,the quaternary ammonium compound is tricapryl methylammonium chloride(“Aliquot 336”) or tetradecylammonium chloride. In examples to follow,the quaternary ammonium salt used was a trialkyl monomethyl ammoniumchloride wherein the alkyl groups contained 8 and 10 carbons, such alkylgroups being straight chained and randomly distributed in the quaternarycation.

The deprotonating anion is capable of being deprotonated by reactionwith the quaternary ammonium salts and include, for example,alpha-hydroxyoximes, benzophenoximes, beta-diketones, fluorinated betadiketones, benzoxazoles, hydroxyquinolines, organophosphoric acids, andnaphthenic acids. As an example, a preferred deprotonating anionincludes diethylhexylphosphate (DEPHA) or Cyanex 272 (Bis(2,4,4,-trimethylpentyl) phosphinic acid). Each of these classes ofcompounds are extensively described in the literature (See e.g., U.S.Pat. No. 4,128,493 and references cited therein) and are readilycommercially available.

Both the quaternary ammonium cation and the deprotonating anion aresoluble in the water-immiscible organic solvent. Preferably, the solventis an aliphatic or aromatic hydrocarbon such as the petroleum derivedliquid hydrocarbons (e.g. kerosene, fuel oil). Other suitable solventsinclude, but are not limited to, chlorinated hydrocarbons.

In addition to the three principal components comprising the QL reagent,a phase modifier, such as di-butyl butyl phosphonate (“DBBP”), can alsobe added to prevent a third phase from forming during the extractionprocess. A preferable phase modifier has both organic and ioniccomponents so that it may be soluble in both phases. Without being boundby theory, acceptable phase modifiers of the present invention shouldnot interfere with the QL reagent by binding to either component.Additionally, it is preferable that the phase modifier maintains aweaker metal binding capacity than the QL reagent and, by itself, doesnot have metal binding capacity. Examples of acceptable phase modifiersinclude DBBP as well as a variety of alcohol-based reagents.

The extraction process may also include multiple stages of usingreciprocating columns. In the first reciprocating column stage, zincdepleted geothermal brine may be mixed in one or more columns with animmiscible organic solvent that contains a specific extracting compoundto extract calcium and iron, while the manganese loads onto the organicphase.

The optimum pH of the solution during the loading step and subsequentscrubbing and stripping steps is between about 2 to about 5. Thetemperature of the solution is preferably in the range of about 180° F.to about 230° F. through this step as well. The concentration of the QLreagent introduced may vary but is preferably between about 0.10M toabout 0.5M, more preferably about 0.15M, about 0.3M, or about 0.45M toachieve maximum manganese extraction of at least 70% with a minimumamount of calcium extraction resulting.

Scrubbing

As the QL reagent is selective against calcium, in one embodimentcalcium may be separated from the organic phase as aqueous calciumraffinate. The organic phase may then be routed to the scrubbing stepfor removal of remaining calcium. The resulting concentration of calciumremaining in the organic phase is usually below about 13 mg/L,preferably below about 10 mg/L, and more preferably below about 2 mg/L.During scrubbing, the remaining calcium is preferably removed from theorganic phase through cation exchange by a combined manganese/hydrogenion displacement mechanism. Preferably, in this step calcium isseparated from the organic phase containing manganese and iron as theprimary metal components, if iron had not already been removed.Preferably above about 90% to about 100% of the calcium is removed fromthe solution in the scrubbing step.

During scrubbing, the loaded organic phase may be processed through theaddition of spent electrolyte, preferably MnCl₂ or another acidicscrubbing solution, preferably with a high manganese concentration whichforms an aqueous phase (between about 40 g/L to about 70 g/L, andpreferably about 50 g/L). In this step, the loaded organic phase may becontacted with the spent electrolyte pre-adjusted to a particular pH ofbetween about 1 to about 5, preferably about pH 4, for the initialintroduction of spent electrolyte. For subsequent addition of spentelectrolyte to the organic phase, the pH of the spent electrolyte ispreferably between about 1 to about 5. Additionally, the oxidationpotential may be adjusted in the spent electrolyte prior to contact withthe organic to between about 300 m/V to about 600 m/V. The reagentsuseful for the pH adjustment and equilibration of the spent electrolyteor the aqueous phase include, but are not limited to, NaOH and HCl.Methods are known in the art that are useful for performingequilibration of the pH during this step.

The contact between the loaded organic phase and the spent electrolytemay occur at a specific organic phase to spent electrolyte ratio (O/A)ranging from about 20:1 to about 1:2, about 15:1, about 10:1, about7.5:1, about 5:1, about 1:1, and preferably between about 20:1 to about5:1. Calcium removal in this step involves a flexible process whereinthe maximum calcium separation may be achieved at O/A ratios of lessthan 20:1.

The scrubbing step may comprise at least one cycle wherein the loadedorganic phase is mixed with the spent electrolyte to form an aqueousphase containing calcium, manganese and iron depending on thecharacteristics of the spent electrolyte and another organic phasecontaining manganese and iron, depending on the characteristics of thespent electrolyte described above. The characteristics of the spentelectrolyte that may affect the consistency of the resulting aqueous andorganic phases include pH, oxidation potential, the introductory O/Aratio and metal concentration. The relevant concentrations of pertinentcomponents may be predicted depending on the characteristics of thespent electrolyte added to the system. The cycles used herein may becontinuous within a closed system such as that existing within a seriesof connected Koch columns. Preferably, the scrubbing step comprisesmultiple cycles between about 2 to about 5 cycles, and more preferablyabout 3 cycles in suitable columns.

In general, two options exist for the organic phase resulting from theaddition and mixing of the spent electrolyte in the scrubbing step.Depending on the embodiment being utilized (as described below), theorganic phase can be routed to another scrubbing cycle wherein anotherside-stream of spent electrolyte is introduced and another scrubbingcycle ensues. The side stream may be similar to the side-stream used inthe previous scrubbing cycle, or it may vary in one or more factors suchas pH, metal concentration, O/A ratio or oxidation potential. The secondoption for the organic phase above is to be routed to a stripping step,as provided below, for extraction of manganese from the organic phaseand reformation and recycling of the QL reagent.

Similarly, options exist for routing of the aqueous phase resulting fromthe present scrubbing step. This phase may be recycled to the feedstream of brine entering the system, routed to an iron removal step orremoved from the system. The spent electrolyte composition and metalconcentration of the resulting aqueous phase discussed below may affectwhere the aqueous phase gets routed to.

Stripping

The stripping step involves treating the organic phase resulting fromthe scrubbing step(s) with a high O/A concentration of an acidicsolution, such as a side stream of acidic spent electrolyte. The pH ofthe spent electrolyte is preferably about 1. The strip solution, afterthe addition of the side stream of spent electrolyte, may be comprisedof about 70% to about 90% spent electrolyte. The manganese metalconcentration in the side stream may be between 40 g/L to about 80 g/L,and more preferably about 50 g/L. Further, the stripping process may beperformed through the addition of HCl, or another non-oxidizing acid,rather than (or in addition to), the introduction of a side stream ofspent electrolyte. If HCl is used, the concentration is preferably inthe range of between 0.05M to about 0.5M. If no spent electrolyte isavailable, then the use of a non-oxidizing acid is the preferred route.The ratio and concentration of either the acid or the side stream ofspent electrolyte introduced to the organic phase solution may varydepending on the amount required to strip the organic phase and reformthe QL reagent. Methods known in the art and illustrated in the Examplessection are useful for determining these concentrations and volumes.

The end product of the stripping step is the formation of an aqueousphase and an organic phase. Producing these phases involves contactingthe organic phase with an aqueous acidic solution and can be illustratedby the following equations (depending on whether a monovalent ordivalent anion is used):

Divalent (e.g., sulfate):L₂M_(org).+Q₂X_(org).+H₂X_(aq).→2LH_(org).+Q₂X_(org).+MX_(aq).

Monovalent (e.g., chloride):L₂M_(org).+2QX_(org).+2H⁺→M²⁺+2LH_(org).+2QX_(org).

As indicated above, at this stage, the metal salt is stripped from theorganic phase and ends up in the aqueous phase. Following this step, theorganic phase can be reused by neutralization and washing (with H₂O) toform the salt of the hydrogen ion exchange reagent and the quaternaryammonium compound (reforming the QL reagent) for use in further metalextractions (See e.g., Example 1). This step may be represented by thefollowing equation:

HL_(org).QX_(org).+NaOH←→QL_(org).+NaX+H₂O

Suitable acids, compounds or gasses (together “compounds”) for use inthis step may be any compound that forms a soluble metal salt withmanganese. Preferably, these compounds should not be oxidizing compounds(i.e., nitric acid, chlorine gas, hydrogen peroxide, oxygen, air, etc.),and they should not interfere with the QL reagent components. Thecompound is preferably hydrochloric acid or sulfuric acid.

When hydrochloric acid is used, this step also may induce calciumremoval, since calcium chloride is formed and precipitates (depending onwhether calcium is removed in prior steps). Alternatively, if sulfuricacid is used instead of hydrochloric acid, an ion exchange orprecipitation step can be added prior to oxidation and electrolysis toremove calcium.

Oxidation

During the steps which may optionally take place prior to the presentoxidation step, a majority of the iron in the brine co-purifies with themanganese. If so, the iron should be removed from the solution toproduce a purified manganese containing solution. The oxidation stepinvolves oxidizing the iron in solution to form an insoluble iron oxideprecipitate which can be removed from solution. The agents used in thisprocess, therefore, must be efficient in oxidizing Fe²⁺ to Fe³⁺ andprecipitating iron by producing iron oxides.

Examples of suitable oxidizing agents include but are not limited tosodium hypochlorite, sodium hydroxide, hydrogen peroxide, and chlorinegas. When sodium hypochlorite (NaOCl) is used, iron precipitates in theform of akaganeite. The oxidizing agent may also be combined with abuffer such as sodium carbonate. In this aspect, the buffer aids inmaintaining the solution pH in the range between about 1.5-5 and alsomay promote the formation of stable precipitates which can be easilyremoved. Seed crystals may also be added to the solution in this step tohelp promote iron precipitation. These seed crystals may be akaganeitecrystals added directly to the strip liquor.

Removal of the iron precipitate can be accomplished using any knownmethod, such as settling, filtration, column precipitation and the like.

In one aspect, the oxidation step may be performed in stages to maximizeakaganeite precipitation. The stages may vary in pH ranges preferablybetween 1.7 to about 1.8, about 2.3 to about 2.4, and about 3.4.Oxidation potential also plays a factor in iron oxidation. Theconcentration and volume of the oxidizing agent may varystoichiometrically. The oxidizing agent concentration may be determinedby routine optimization.

After the addition of the oxidizing agent and precipitation of iron, thesolution should be iron depleted. The concentration of iron remaining insolution may be in the range of about 10% to less than 1%, about 8%,about 5%, about 3%, and preferably about or below 1%. Further, oxidationaccording to the above description may take place in a series of cascadearranged tanks (See FIG. 6).

After oxidation and precipitation of iron oxides, the iron depletedsolution may be further purified by a manganese solvent extraction stepwherein remaining iron and other trace metals such as copper and otherbase metals, which may still exist in solution, are removed. Thisprocess preferably involves the addition of a hydrogen exchange reagentsuch as DEPHA or a Cyanex extractant to the solution. The addition ofthe extractant causes the formation of FeCl₂ (and other metal chlorides)which may be removed. The purified manganese solution may then be routedto electrowinning. The precipitated iron for this step may be recycledto the beginning of the oxidation step for further use as seed materialto aid precipitation.

FIG. 7 presents a schematic of the process steps that may be used foriron removal.

Electrowinning

To extract electrolytic manganese dioxide from the product of the priorsteps, the iron depleted aqueous phase (containing manganese salt) isexposed to an electric current between an anode and cathode. In oneembodiment, the anode is composed of a corrugated titanium plate and thecathode is composed of one or more graphite plates or slabs. The currentwill cause the manganese dioxide to deposit on the anode. The irondepleted aqueous phase (“bath”) may contain a manganese concentrationbetween about 40 gms/kg to about 70 gms/kg, and usually contains atleast about 50 gms/kg, as well as about a 50 gms/kg concentration ofhydrochloric acid. The primary electrolyte for use in the presentelectrowinning step is hydrochloric acid. The disclosed process involvesa chloride route to the electrowinning step. However, the manganesesalts of manganese chloride and manganese sulfate may be used in theelectrowinning process to produce manganese dioxide (sulfuric acid wouldbe used instead of hydrochloric acid if manganese sulfate is themanganese salt here). However, one of these compounds may be preferredover the other, depending on how the manganese will be usedcommercially. Therefore, a further embodiment of the present inventionrelates to converting manganese chloride to manganese sulfate.

Optional Product Finishing

Following the electrowinning step, the manganese dioxide productdeposited on the anode is optionally finished to produce a commercialproduct. This process involves the following steps. First, the anodeplates one removed from the electrowinning bath and washed with anaqueous solution (e.g., hot water) to remove any residual acid. Theseshould be allowed to dry. The manganese dioxide may then be removedusing mechanical forces such as agitation, flexing, scraping, etc.Finally, the manganese dioxide may be ground, milled and neutralizedusing known methods. For example, it may be dry ground in a C-E Raymondring-roller mill with air classifier to a nominal ˜100 um.

The neutralization step may involve the following: first, a manganesedioxide slurry is made through the addition of water and a base to bringthe pH of the slurry to equal to or greater than 6, since washing theplates during product finishing only removes acid from the exposedsurface areas. Accordingly, this step removes occluded and adsorbedacid. Then the final slurry product may be filtered and dried.

The final manganese product is preferably comprised of at least 99% puremanganese dioxide.

Exemplary Method #1

In one embodiment (See FIG. 1), zinc depleted brine is routed to aseries of reciprocating Koch columns involving about 6 stages ofcolumns. In a loading phase, QL reagent is then introduced to the brinein one or more columns in a counter current exchange method. The countercurrent exchange preferably extracts iron, calcium and manganese. Twophases may then be formed comprising an organic phase containingmanganese, iron and calcium as the principal components and the aqueousphase containing calcium as the principal component. The aqueous phasemay then be separated from the organic phase and removed form thepresent system. The organic phase is then preferably scrubbed throughthe introduction of about 0.1 M HCl in an organic to acid ratio of about6:1. The introduction of the HCl produces two phases, organic andaqueous wherein the principal components of the former organic phase aredisplaced into a new aqueous phase. In this second set of organic andaqueous phases, the aqueous phase contains iron, manganese and calcium.The organic phase is then routed back to the loading stage.

The aqueous phase may then be routed to an oxidation stage for ironremoval. In one embodiment, NaOCl is introduced as the oxidizing agentcausing the iron to fall out of solution in the form of insoluble ironor akaganeite. The oxidizing process may preferably occur in severalstages of cascade-arranged tanks (see FIG. 6). In these tanks, understrictly controlled oxidation potential (about 600 mV) and pH (pHpreferably of about 1.5, about 1.75, about 1.75, or about 3.4, dependingon the tank), iron will be oxidized from soluble ferrous to an insolubleferric form. The time in each tank in the cascade-arranged series mayvary, but is preferably between about 15 minutes to about 60 minutes ineach tank. It is preferable in this embodiment to use pressurefiltration to separate the solid iron precipitate from the aqueousphase. After removal of solid iron from the aqueous phase, the aqueousphase may then be routed to a manganese solvent extraction step. Theiron precipitate may then be recycled for use as additional seedmaterial for further iron precipitation.

As provided above, solvent extraction is accomplished through theintroduction of DEPHA or a Cyanex extractant to the aqueous phase. Theseextractants will cause metals such as soluble iron to form FeCl₂. TheFeCl₂ may then be stripped from the extractant through the addition ofwater. The resulting solution may then be routed to a manganesereduction step involving the introduction of free state manganese. Atthis point in the process, calcium may still exist in solution. Toremove remaining calcium another loading stage may be undertaken whereinQ is added in an O/A ratio of about 1:1. In this step, the remainingmanganese is loaded onto the organic phase, displacing calcium whichforms CaCl₂ in an aqueous phase which may be removed from the system.The manganese is then stripped from the organic phase through theaddition of a side stream of spent electrolyte from electrowinningcontaining a concentration of about 50 g/L manganese at a pH of about 1.

The resulting manganese containing solution may then be routed to anelectrowinning step wherein manganese dioxide is deposited. Themanganese dioxide is then preferably refined into about 95% to about100%, about 97%, about 98%, and preferably about 99% pure manganesethrough the product finishing steps described above.

Exemplary Method #2

In another embodiment (See FIG. 2), the extraction process requiresabout 6 stages of reciprocating columns. In the first reciprocatingcolumn stage, zinc depleted geothermal brine is mixed in one or morecolumns with an immiscible organic solvent that contains a specificextracting compound (QL reagent). Herein, a counter current extractionof manganese, calcium and iron takes place and the manganese loads ontothe organic phase. The organic loaded phase is then preferably scrubbedin more that one column through the addition of a small side-stream ofspent or recycled electrolyte, which is preferably manganese-rich, toaid in the displacement of calcium. This side stream may be introducedin a concentration of about 10% spent electrolyte to about 90% organicphase at a pH of about 4 and containing a manganese concentration ofabout 50 g/L. The pH of the side stream may preferably be adjustedthrough known methods, e.g., the addition of NaOH, to the desired pHprior to introduction to the organic phase. The spent electrolyte may beobtained from the electrowinning step. Depending on the manganesecontent, the aqueous phase that forms as a result of the addition of theside stream of spent electrolyte may be either discarded or recycled tothe brine feeding the column for further loading. A measurable manganesecontent usually indicates that recycling is preferable. This scrubbingstep preferably depletes the organic phase of calcium.

Subsequent to scrubbing, the scrubbed organic phase may be routed to astripping step useful for stripping the manganese and iron from theorganic phase and into the aqueous chloride phase. This step maycomprise the introduction of spent electrolyte to the scrubbed organicin a concentration of about 90% spent electrolyte to about 10% organicphase. Preferably, the spent electrolyte is at a pH of about 1 and themanganese concentration of the electrolyte is about 50 g/L. At thispoint, the QL reagent used in the loading step is reformed throughneutralization and washing to form the salt of the hydrogen ion exchangereagent and the quaternary ammonium compound (reforming the QL reagent)for use in further metal extractions. The QL reagent may be reused manytimes, usually over about 500 times, through the disclosed reformationmethods.

Further to the stripping step, the aqueous phase may preferably berouted to a step for iron removal comprising oxidation and manganesechemical extraction. Other embodiments of these steps are discussedabove. When iron is removed from the aqueous phase and the solutioncontains an appropriate concentration of manganese for electrowinning(see above), the chloride solution containing manganese is routed to theelectrowinning step wherein manganese dioxide is deposited.

Exemplary Method #3

In another embodiment (See FIG. 3), the loaded organic phase treatmentmay be the following. The loaded organic phase is scrubbed with a smallbleed stream (5-15%) of spent electrolyte (pre-neutralized to pH 4) todisplace calcium. Depending on the manganese content of the bleed, itcould be either discarded or recycled to the brine feed to the QLsolvent extraction. The organic phase is preferably depleted of zinc atthe end of this scrub. In this embodiment, the scrubbed organic is thenscrubbed again with spent electrolyte introduced at a concentration ofabout 15-25% spent electrolyte at a neutralized pH of about 5. Thescrubbed organic phase is then routed to a stripping step for a finalremoval of manganese and reformation of the QL reagent. The aqueousphase is then routed to oxidizing and manganese extraction steps wherethe iron precipitates as akaganeite and FeCl₃ and a manganese chloridesolution results. The manganese chloride solution may then be combinedwith the aqueous phase containing manganese resulting from the strippingstep discussed above. Further, these two solutions may be combined toform the electrowinning bath. This combined solution may be used inelectrowinnning to form manganese dioxide. As a final step, productfinishing may be undertaken in accordance with the steps describedabove.

Exemplary Method #4

In a further embodiment (See FIG. 4), the same preliminary loading andscrubbing steps are utilized to remove calcium from the loaded organic.In addition, in this embodiment a second scrubbing stage is included inwhich the loaded organic is treated with a small bleed stream (10-20%)of spent electrolyte that has been reduced to ˜300 mV with manganesemetal, and the pH adjusted (if necessary) to ˜pH 1. In this scrubbingstage, iron is stripped from the organic because of the low pH, butmanganese remains loaded (switching from DEPHA to Aliquot 336) becauseof the formation of anionic manganese chloride. Depending on themanganese content of the scrub solution, it would either be discarded ortreated to separate iron and manganese.

Further to the second scrubbing, the aqueous phase may be routed to thesteps described for iron removal and the organic phase may be routed toa stripping step for stripping of the remaining manganese from theorganic and reformation of the QL reagent. In this embodiment, thestripping step involves contact of the organic phase with aconcentration of about 60-80%, preferably 70%, spent electrolyte at a pHof about 1, a manganese concentration of about 50 g/L and an EMF ofabout 600 mV. The aqueous phase resulting from contact of the spentelectrolyte with the organic phase which contains manganese should thenbe routed to the electrowinning step for manganese dioxide production.

Exemplary Method #5

In a still further embodiment (See FIG. 5), iron is removed from thebrine solution prior to loading of the QL reagent. In this embodiment,NaOCl is contacted with the brine solution thus causing akaganeiteformation and precipitation from solution. This akaganeite residue maythen be removed through methods known in the art, such as pressurefiltration. The iron depleted brine solution is then routed to QLloading where manganese and calcium load onto the organic. The aqueousphase thus formed containing an elevated concentration of calcium maythen be discarded. Calcium may then be scrubbed from the organic phasethrough the introduction of about 5-10% spent electrolyte (at a pH ofabout 1). The aqueous phase may then be routed back for further QLloading or discarded, depending on the concentration of manganese inthis solution. The organic phase may then be routed to a stripping stepwherein about 90-95% spent electrolyte (at a pH of about 1) is contactedwith the organic phase, the manganese is then displaced into the aqueousphase and the QL reagent may then be reformed through neutralization andwashing. In this embodiment, the aqueous phase may then requireadditional pH adjustment, through the addition of NaOH, to place thesolution in a proper pH for electrowinning. The solution may then berouted to electrowinning and optional product finishing.

All references cited herein are hereby incorporated by reference intheir entireties, whether previously specifically incorporated or not.As used herein, the terms “a”, “an”, and “any” are each intended toinclude both the singular and plural forms.

Having now fully described this invention, it will be appreciated bythose skilled in the art that the same can be performed within a widerange of equivalent parameters, concentrations, and conditions withoutdeparting from the spirit and scope of the invention and without undueexperimentation.

While this invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodifications. This application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth.

We claim:
 1. A method of depleting iron from a geothermal brine solutioncomprising: (a) contacting the geothermal brine solution with anoxidizing agent, wherein said oxidizing agent oxidizes soluble iron inthe solution to form an insoluble iron oxide precipitate; and (b)removing the iron oxide precipitate from the solution to form aniron-depleted geothermal brine solution.
 2. The method of claim 1,wherein the soluble iron is oxidized from an oxidation state of Fe²⁺ toFe³⁺.
 3. The method of claim 1, wherein the oxidizing agent is sodiumhypochlorite, sodium hydroxide, hydrogen peroxide, or chlorine gas. 4.The method of claim 1, wherein the oxidizing agent is sodiumhypochlorite.
 5. The method of claim 4, wherein the insoluble iron oxideprecipitate is iron akageneite.
 6. The method of claim 1, wherein theoxidizing agent is combined with a buffer.
 7. The method of claim 6,wherein the buffer is sodium carbonate.
 8. The method of claim 1,wherein the pH of the geothermal brine solution remains in the rangebetween about 1.5 and
 5. 9. The method of claim 8, wherein the oxidizingis performed over at least two stages of increasing pH.
 10. The methodof claim 9, wherein a first stage is performed at a pH between about 1.7and 1.8, a second stage is performed at a pH between about 2.3 and 2.4,and a third stage is performed at a pH of about 3.4.
 11. The method ofclaim 1, wherein the iron-depleted geothermal brine solution has asoluble iron concentration of less than about 10%.
 12. The method ofclaim 11, wherein the iron-depleted geothermal brine solution has asoluble iron concentration of less than about 8%.
 13. The method ofclaim 12, wherein iron-depleted geothermal brine solution has a solubleiron concentration of less than about 5%.
 14. The method of claim 13,wherein iron-depleted geothermal brine solution has a soluble ironconcentration of less than about 3%.
 15. The method of claim 13, whereiniron-depleted geothermal brine solution has a soluble iron concentrationof about 1%.
 16. The method of claim 1, further comprising: (c)contacting the iron-depleted geothermal brine solution with a QL reagentto create a reaction solution, wherein the QL reagent comprises aquaternary ammonium compound, a hydrogen ion exchange reagent and anorganic solvent, wherein the reaction solution comprises an organicphase and an aqueous phase, and wherein the reaction solution ismaintained at a temperature in the range of about 180° F. to about 230°F.; and (d) extracting one or more non-manganese impurities into theaqueous phase, wherein the organic phase becomes an impurity depletedsolution; and (e) separating the aqueous phase from the organic phase.17. The method of claim 16, wherein the non-manganese impurity iscalcium.
 18. The method of claim 16, wherein the non-manganese impurityis zinc.
 19. The method of claim 16, further comprising: (f) extractingmanganese from the impurity depleted reaction solution, wherein the pHof the solution remains constant.
 20. The method of claim 16, furthercomprising: (f) removing any remaining calcium from the impuritydepleted solution by extracting the impurity depleted solution with anaqueous acidic scrubbing solution.
 21. The method of claim 20, whereinabout 90% to about 100% of calcium is removed from the impurity depletedsolution in the scrubbing step.
 22. A method of depleting calcium from ageothermal brine solution comprising: (a) contacting the geothermalbrine solution with a QL reagent to create a reaction solution, whereinthe QL reagent comprises a quaternary ammonium compound, a hydrogen ionexchange reagent and an organic solvent, wherein the reaction solutioncomprises an organic phase and an aqueous phase, and wherein thereaction solution is maintained at a temperature in the range of about180° F. to about 230° F.; (b) extracting calcium into the aqueous phase,wherein the organic phase becomes a calcium-depleted geothermal brinesolution; and (c) separating the aqueous phase from the organic phase.23. The method of claim 22, further comprising: (d) removing anyremaining calcium from the calcium-depleted geothermal brine solution byextracting the calcium-depleted geothermal brine solution with anaqueous acidic scrubbing solution to form a scrubbed organic phase. 24.The method of claim 23, wherein the scrubbing solution is amanganese-rich scrubbing solution.